A successor communication method to W-CDMA and HSDPA, i.e., Long Term Evolution (LTE) (also called Evolved UTRA and UTRAN or Super 3G), is currently being discussed by 3GPP that is a standardization group for W-CDMA. In 3GPP, orthogonal frequency division multiplexing (OFDM) has been proposed as a downlink radio access method and single-carrier frequency division multiple access (SC-FDMA) has been proposed as an uplink radio access method (see, for example, 3GPP TR 25.814 (V7.0.0), “Physical Layer Aspects for Evolved UTRA,” June 2006).
In OFDM, a frequency band is divided into multiple narrower frequency bands (subcarriers) and data are transmitted on the subcarriers. The subcarriers are densely arranged along the frequency axis such that they partly overlap each other but do not interfere with each other. This approach enables high-speed transmission and improves frequency efficiency.
In SC-FDMA, a frequency band is divided into narrower frequency bands and the narrower frequency bands are allocated to different terminals for transmission. This approach makes it possible to reduce interference between terminals. Also, SC-FDMA reduces variation of the transmission power and therefore makes it possible to reduce power consumption of terminals and to achieve wide coverage.
In a frequency scheduling method that employs propagation path fluctuations in the frequency domain caused by frequency selective fading, data are transmitted using frequency bands with good reception conditions and therefore user terminals are requested to send wideband signals for the measurement of received-channel quality. For example, in E-UTRA uplink, transmission bands are allocated to data channels taking into account the frequency selectivity of received channels and each user terminal (UE) is requested to send a wideband pilot signal called a sounding reference signal (SRS) for measuring the uplink received channel quality.
However, when a UE located away from the base station is to transmit a wideband sounding reference signal, the transmission power is limited. Therefore, the received power of the sounding reference signal at the base station becomes low and the accuracy in measuring the received channel quality is reduced.
In a method proposed to cope with this problem, the transmission bandwidth for the sounding reference signal is adaptively adjusted according to the distance, i.e., a path loss, between the UE and the base station. Meanwhile, the following two methods for multiplexing sounding reference signals with different bandwidths have been proposed:                Multiplexing method using distributed FDMA (FIG. 1)        Multiplexing method where sounding reference signals are classified into groups by bandwidths and the grouped signals are multiplexed using localized FDMA (FIG. 2).        
In both methods, sounding reference signals with the same bandwidths are multiplexed by CDMA.